Renewable energy and products are playing an increasingly important role in today's world. Valuable products, such as methane, can be produced from renewable organic materials, such as biomass, by biological conversion processes.
Anaerobic digestion has been recognized to be able to stabilize sludge and other predominantly organic materials, and produce usable product gas of varying composition. Anaerobic digestion uses a consortium of natural bacteria to convert organic waste to carbon dioxide and methane in the absence of oxygen, which is accomplished by a consortium of microorganisms working synergistically, involves four steps, namely hydrolysis, acidogenesis, acetogenesis, and methanogenesis, of which hydrolysis is the most rate-limiting stage.
Anaerobic digestion systems can be separated into two major types, one phase systems and two phase systems. In one phase systems, acidogenesis and methanogenesis microorganisms are housed in the same vessel. In the two phase systems, acid-forming phase is separated from a methane-forming phase, so that each phase can be operated more efficiently. In the first phase, organic substrate is liquefied and broken-down into lower molecular weight and other intermediates which are converted to methane in the second phase.
In conventional liquid anaerobic digestion (AD) facilities, the digesters are operated with a total solid content of 0.5% to 15%. Typical liquid AD produces a large amount of effluent which normally contains a high amounts of ammonium, phosphate, suspended solids and dissolved solids, has been applied as fertilizer. However, there is a limit to the distance that this effluent can be transported and applied economically, and there is often not enough farmland in the adjoining territory to make use of the total effluent. Additionally, excessive application of post-digestion effluent on agricultural land has resulted in nutrient overloading and sediment pollution in local watersheds.
Thus, the effluent of liquid AD is often treated prior to discharge creating an economic barrier to the use of this technology both as a waste management tool and as an energy production tool. Solid-liquid separation followed by membrane separation of the effluent produces an organic fertilizer and clear water, however, this process is costly and flocculants are generally employed to increase the efficiency of the separation. These flocculants create their own problems as un-reacted acrylamide monomers remaining in the added polymers have been shown to be carcinogenic, although the polymers themselves are harmless. In addition, the polymers are non-biodegradable posing another hurdle for the use of the effluent as a fertilizer. In all, the handling of the effluent of liquid AD is a major hurdle for the use of liquid AD.
In contrast to liquid AD systems, solid state anaerobic digestion (SS-AD) systems operate with 15-40% total solids, making them suitable for processing the organic fraction of municipal solid waste (MSW). SS-AD systems have several advantages over liquid AD systems including: smaller volume; less energy consumption for heating; no processing energy for stirring; and reduced material transportation costs. Due to the lower water content, the digestate of SS-AD process can be used as fertilizer or pelletized for use as a fuel, making the residue of the process much easier to handle than that of the liquid AD processes.
Disadvantages of conventional SS-AD systems include the large amount of inoculum required for efficient digestion, longer retention time (three times that of liquid AD), and the requirement of nitrogen nutrients supplementation when lignocellulosic biomass is used.
If operational conditions (pH, C:N ratio, solid content, and temperature) are not maintained at optimal values, imbalances among microorganisms can lead to digester upset (failure) as a result of buildup of organic acids which can inhibit the methanogen activity.